An electron beam has a strong interaction with a substance; and, in order to be used as a probe, must be accelerated at a high voltage and propagated in vacuum. For that reason: it is impossible to configure an optical system on an optical bench by using a three-dimensional space like a laser optical system; and the system is configured by one-dimensionally arraying a source (an electron source), a lens, a specimen, and an observation device in one vacuum chamber (one mirror body). Because of such a configuration, with a conventional electron microscope, it is very difficult to observe a specimen simultaneously from two directions and even an interference type electron microscope that requires two electron beams is handled within the range of an optical system configured one-dimensionally and an interference figure recorded as a result of interference is also one image at one time.
As three-dimensional observation or stereoscopic observation with such an electron microscope, tomography, a stereo method, confocal scanning imaging, or the like is adopted. The stereo method: is the oldest observation method and a method of obtaining two images (or two specimen images) by inclining a specimen to an electron beam at angles of several degrees in the plus and minus directions and sterically observing the specimen with a stereo viewer or the like; and is disclosed in Michio Kiritani, “Electron Microscope”, Vol. 16, (1981) P 71-P 81 for example. In the stereo method, the two images of different inclination angles correspond to a right visual field image and a left visual field image respectively and the angle difference between the two images comes to be a parallactic angle.
Tomography is a method of, following recent development in image processing technology with a computer and computer control of specimen location/angle, obtaining a plurality of (several tens of) images while the specimen inclination angle is changed by several degrees in the range of ±70 degrees for example in an identical visual field, processing the obtained images with a computer, thereby reconstructing a three-dimensional structure in a prescribed visual region of the specimen in the computer, and making it possible to sterically observe the specimen from arbitrary directions. The tomography is disclosed in K. Kimura et al., J. Electron Microsc., Vol. 54(4), (2005) P 373-P 377 for example and it may be regarded as a technology developed from the stereo method with a high degree of accuracy.
The above methods do not depend on a method for obtaining a image and hence can be adopted not only in a transmission electron microscope but also in a scanning transmission electron microscope. Further, the stereo method: is also adopted in a scanning electron microscope the main observation object of which is the surface appearance of a specimen; and is disclosed in JP-A-Hei6 (1994)-310070 for example. In JP-A-Hei6 (1994)-310070, it is so designed as to observe a specimen from various angles by: deflecting an electron beam with a deflector or the like; switching between a vertical irradiation electron beam and an oblique irradiation electron beam; and alternately irradiating the specimen with the two beams. Likewise, the stereoscopic observation method of a specimen with a transmission electron microscope is disclosed also in JP-A-2004-171922. JP-A-2004-171922 discloses a method of reconstructing a sterical shape and displaying the sterical shape on a monitor by: alternately separating a first electron beam and a second electron beam deflected to different angles from an electron beam generated from an electron beam source by switching the polarity of voltage applied to an electron trapezoid prism; irradiating a specimen alternately with the first and second electron beams; and thereby taking a first image and a second image.
The confocal scanning imaging is a kind of the scanning transmission electron microscope and a method of obtaining the three-dimensional structure of a specimen from a plurality of images taken by changing the observation plane in the optical axis direction by: reducing the convergence portion of an electron beam in the optical axis direction with the electron beam of a large irradiation angle; and producing a two-dimensional image of a shallow field depth with a small-diameter stop in an imaging system. The confocal scanning imaging is disclosed in JP-A-2008-270056 for example.
Meanwhile, an interferometer equipped with a two-stage biprism having a structure wherein an electron biprism on the upper stage is disposed on the downstream side of a specimen and an electron biprism on the lower stage is disposed in a space at the shadow of the upper-stage electron biprism is disclosed in JP-A-2005-197165.
Then, an example of an interferometer using a quadrangular-pyramid electron beam prism in a condenser optical system is disclosed in FIG. 15 of JP-A-2006-313069.